The present invention relates to a process for preparing compositions which are useful as intermediates for dispersants used in lubricating oil compositions or as dispersants themselves. In addition, some of the compositions prepared by the present process are useful in the preparation of high molecular weight dispersants which have superior dispersant properties for dispersing sludge and varnish and superior Viton Seal compatibility. Such high molecular weight dispersants also advantageously impart fluidity modifying properties to lubricating oil compositions which are sufficient to allow elimination of some proportion of viscosity index improver from multigrade lubricating oil compositions which contain these dispersants.
It is known in the art that alkenyl-substituted succinic anhydrides have been used as dispersants. Such alkenyl-substituted succinic anhydrides have been prepared by two different processes, a thermal process (see, e.g., U.S. Pat. No. 3,361,673) and a chlorination process (see, e.g., U.S. Pat. No. 3,172,892). The polyisobutenyl succinic anhydride ("PIBSA") produced by the thermal process has been characterized as a monomer containing a double bond in the product. Although the exact structure of chlorination PIBSA has not been definitively determined, the chlorination process PIBSA materials have been characterized as monomers containing either a double bond, a ring other than a succinic anhydride ring and/or chlorine in the product. [See J. Weill and B Sillion, "Reaction of Chlorinated Polyisobutene with Maleic Anhydride:Mechanism Catalysis by Dichloromaleic Anhydride", Revue de l'Institut Francais du Petrole, Vol. 40, No. 1, pp. 77-89 (January-February, 1985).] Such compositions include one-to-one monomeric adducts (see, e.g., U.S. Pat. Nos. 3,219,666; 3,381,022) as well as adducts having polyalkenyl-derived substituents adducted with at least 1.3 succinic groups per polyalkenyl-derived substituent (see, e.g., U.S. Pat. No. 4,234,435).
In addition, copolymers of maleic anhydrides and some aliphatic alpha-olefins have been prepared. The polymers so produced were useful for a variety of purposes including dispersants for pigments and intermediates in the preparation of polyesters by their reaction with polyols or polyepoxides. However, olefins having more than about 30 carbon atoms were found to be relatively unreactive. (See, e.g., U.S. Pat. Nos. 3,461,108; 3,560,455; 3,560,456; 3,560,457; 3,580,893; 3,706,704; 3,729,450; and 3,729,451).
Commonly assigned copending U.S. patent application Ser. No. 251,613, to James J. Harrison, filed Sep. 29, 1988, entitled "Novel Polymeric Dispersants Having Alternating Polyalkylene and Succnic Groups" discloses copolymers prepared by reacting an unsaturated acidic reactant, such as maleic anhydride, with a high molecular weight olefin, such as polyisobutene, in the presence of a free radical initiator, wherein at least about 20 percent of the total high molecular weight olefin comprises an alkylvinylidene isomer and wherein the high molecular weight olefin has a sufficient number of carbon atoms such that the resultng coolymer is soluble n lubricating oil. In U.S. Ser. No. 251,613, it is also taught that the reaction may be conducted neat or in the presence of a solvent in which the reactants and free radical initiator are soluble. Suitable solvents disclosed in U.S. Ser. No. 251,613 include liquid saturated or aromatic hydrocarbons having from 6 to 20 carbon atoms, ketones having from 3 to 5 carbon atoms and liquid saturated aliphatic dihalogenated hydrocarbons havng from 1 to 5 carbon atoms. Examples of solvents taught in U.S. Ser. No. 251,613 are acetone, tetrahydrofuran, chloroform, methylene chloride, dichloroethane, toluene, dioxane, chlorobenzene and xylene.
The use of halogenated hydrocarbons as a solvent in the reaction of unsaturated acidic reactants, such as maleic anhydride, and high molecular weight olefins of the type described in U.S. Ser. No. 251,613 has a number of disadvantages. Such solvents are expensive, they are environmentally undesirable and they impede recycling of lubricating oils because of the residual halogen content.
In the above-described reaction, the solvent is used primarily to solubilize the unsaturated acidic reactant, but also serves to reduce the viscosity of the reaction mixture. Unsaturated acidic reactants such as maleic anhydride are not very soluble in high molecular weight olefins at typical reaction temperatures of 50.degree. C. to 210.degree. C. When the unsaturated acidic reactant is maleic anhydride, it has been found that if the maleic anhydride forms a separate phase due to poor solubility, not only is the reaction rate negatively affected, but an undesirable resin or tar-like substance is formed which is believed to be polymaleic anhydride. Consequently, it would be highly advantageous to provide a process which avoids this condition, without having to resort to a halogenated hydrocarbon solvent.